The present invention relates to a microwave resonator for an EPR (=“electron paramagnetic resonance”) probe head comprising: a metal cavity body supporting an electromagnetic microwave resonance mode, the microwave mode having an even number of local maxima of microwave electric field energy, at least one opening for inserting a sample tube to a center position of the resonator, the center of the opening and the center position of the resonator defining an x-axis, at least one opening for transmitting microwave radiation into the resonator, at least two identical dielectric elements located symmetrically to the plane known as “E-field nodal plane” which contains the x-axis and a z-axis perpendicular to the x-axis.
A device of this type is known from U.S. Pat. No. 3,757,204.
In the EPR method it is often desirable or even necessary to perform measurements at variable temperature conditions on a sample. The apparatus to irradiate an unknown paramagnetic sample with a microwave field is called EPR probe head. For easiness of achieving variable temperature conditions the probe head is usually placed inside of a cryostat which is coaxial to the EPR sample insertion mechanism. The cryostat itself is placed either between the poles of a split-coil magnet or inside the bore of a solenoid magnet.
For EPR experiments at X-band (8-12 GHz) or higher frequencies the size of magnet gap or solenoid bore becomes an important factor in the ownership cost of the EPR system. Such a size constraint also directly affects the design and performance of EPR probe heads. For example, at X-band or lower frequencies the standard air-filled EPR probe heads do not fit inside usual cryostats. It is necessary to use coaxial transmission lines instead of waveguides for microwave transmission while for the microwave resonator, as part of the probe head, this situation has been mitigated in so far by building loop-gap resonators (see e.g. DE 33 00 767 A1 for Split-Ring and BLGR solutions for Flex Line probe heads) or cylindrical shaped dielectric loaded resonant cavities (see e.g. DE 41 25 655 A1 or DE 41 25 653 A1 for Sapphire cylindrical TE011 mode solution for Flex Line probe heads).
In electron-nuclear dual spin resonance experiments, like ENDOR or EPR-DNP, it is often needed to simultaneously optimize the efficiency of microwave and RF field application over the sample volume. Fulfilling the constraint of optimal EPR and respectively NMR functionalities directly affects the design and performance of the apparatus.
Another application, the EPRI (EPR Imaging) method, requires the presence of a multiple sets of coils to create a magnetic gradient at the unknown sample inside the EPR microwave cavity. The fixture for these coils should be as mechanically decoupled as possible from the sensitive parts of EPR microwave cavity. This is highly difficult to accomplish due to space constraints if the cavity has circular symmetry.
It is particularly interesting to observe the influence of experimental conditions, in particular the temperature, on the microwave concept for EPR probe heads.
For high-sensitivity EPR applications at room temperature conditions the state-of-the-art EPR probe heads include flat shaped, air-filled microwave cavities (see e.g. DE 33 00 767 A1, DE 41 25 655 A1, DE 41 25 653 A1, U.S. Pat. No. 3,931,569-A for rectangular TE102, cylindrical TM110 and Reentrant mode probe heads).
For variable temperature experimental conditions, the state-of-the-art EPR probe heads are built around either via loop-gap or dielectric loaded resonating cavities, especially at L, S, C and X-bands in order to decrease the size of the resonating cavity.
The three versions of flat resonator geometries cited above traditionally used at room temperature are missing here. Instead one can find solutions based on dielectric loaded resonators or loop-gaps having circular symmetry along sample length direction. Indeed, such a choice for resonating mode symmetry is particularly suited to optimize the EPR side of the microwave problem, the filling factor parameter being considered in so far the most important. Attempts to use a coaxial stack of two or more dielectric elements for cavity loading have also been considered but proved to be more difficult than helpful. Even more, for an increasingly larger number of modern EPR applications, the microwave cavity geometries using a single element for dielectric loading also present serious challenges in obtaining a suitable trade-off between performance and usage stability.
It is an object of the invention to achieve performance and quality enhancements of EPR, EPR-ENDOR/DNP and EPRI variable temperature probe heads. This is realized by flat geometry resonators based on dielectric loaded microwave rectangular TE012 or cylindrical TM110 resonant modes with dielectric elements having a thickness comparable to the outer diameter of EPR sample tubes. A full disclosure of the conceptual design for this new microwave cavity for EPR purposes at variable temperature conditions or for narrow magnet gaps is the focus of the present invention.
U.S. Pat. No. 3,122,703 describes a cooled microwave resonator for EPR. A notched Dewar is used to cool an air-filled rectangular TE102 or cylindrical TM110 resonator.
U.S. Pat. No. 3,931,569 shows in FIGS. 2 and 5 (see present FIGS. 8A and 8B, respectively) standard air-filled rectangular TE102 and cylindrical TM110 resonators for EPR.
U.S. Pat. No. 3,757,204 describes different configurations of microwave resonators employing dielectric material for improving RF field uniformity along the sample. Especially FIG. 8C (taken from FIG. 4 of U.S. Pat. No. 3,757,204) depicts a microwave resonator with two dielectric plates extending to either side of the sample.
As the purpose of U.S. Pat. No. 3,757,204 is to homogenize the electromagnetic RF field in the sample, the dielectric plates are located close to the sample as shown in present FIG. 8C. As indicated in present FIG. 8D (taken from FIG. 1 of the document U.S. Pat. No. 3,757,204) the electric field E of the resonant mode has two maxima left and right to the center of the sample. To achieve its claimed functions, the dielectric plates in FIG. 8C must not extend into the regions with maximum of electric field but must increase their extensions in the peripheral regions having less electric field. As claimed, for such purposes a first requirement is an overall concave geometry for the inserts. Even more, a second implicit requirement is to maintain the homogenization of the RF field when a general sample with various dielectric properties is in the resonator. Whereas in the vertical direction the dielectric inserts have the same size as the sample and the resonator, along the shortest side of the resonator they are longer compared to the sample but shorter compared to the resonator length.
However, it is known that higher filling factors in EPR resonators require a trade-off in the Q-factor of the cavity, which linearly determines the EPR signal intensity from the sample. It is also known that limiting the sample volume requires a linear trade-off in the EPR signal intensity. These two trade-offs may cancel the advantages brought in by the claimed increase of the filling factor and a trade-off in the application range for this technical solution occurs. In yet another particular aspect, considering the class of high-sensitivity low-background EPR probe heads at X-band, usage of this approach in U.S. Pat. No. 3,757,204 does not provide sufficient reduction of resonator size to allow to be used in cryostats with standard 2″ access bore, whereas any attempt to decrease the resonator size via dielectric loading using known low-background dielectrics will decrease or cancel the positive effect expected from the above claimed assumptions.
The present invention describes a way to substantially overcome one or more disadvantages and trade-offs of the above discussed existing methods.
One major object of the present invention is to propose a high sensitivity EPR resonator, with low background signals, that achieves a small size, compatible with narrow gap (<2 cm) magnets or cryostats.
Another object of the present invention is to propose an EPR resonator with high efficiency of static or low frequency field irradiation of an EPR sample.